This invention relates to marine process vessels adapted for subsea work. In particular, it relates to a novel surface facility for installing and maintaining underwater flowlines for oil and gas production from offshore wells.
After a subsea well has been completed and a wellhead or group of wellheads are to be connected in fluid flow to the water surface through a production riser, a surface facility is required to receive the hydrocarbon fluids for processing or transfer to other transport vessels. When this operation is carried out in open sea or otherwise subject to significant variations in tides, currents and weather conditions, a compliant riser system may be employed to establish fluid communication between the surface vessel and subsea locations. Installation of such a compliant riser system can require handling of flexible conduits, which serve as oil and gas flowlines, as hydraulic control lines, and as service lines between the surface facility and the wellheads. Such flowlines contain petroleum and/or gas at high pressure and are subject to abrasion and entangling from currents and waves.
In a typical compliant riser system, a relatively fixed lower riser section extends from the marine floor to a submerged location below a zone of turbulence, at which point it may terminate with a buoy section. Between the buoyed lower riser section and the surface facility, a flexible flowline bundle can be utilized to accommodate the vertical fluctuations, currents, etc., which may result in large lateral excursions of the surface facility relative to the lower riser section as well as heaving action due to waves and tides.
The lower riser section may be installed by a floating drilling vessel or semi-submersible rig in a known manner, but major installations, repairs, and replacement operations, such as installation of the flexible flowline bundle portion and maintenance thereof by replacement of individual conduits, require bringing additional specialized vessels to the scene. There is consequently increased risk of multiple collision between large vessels, increased cost, and a likelihood that personnel who are unfamiliar with the compliant riser system will be involved in the operation.
This situation has therefore created a need for a surface vessel that is capable of continued functioning as a workship as well as a processing and/or fluid handling unit. Such self-contained service capability provides faster on-site response, least cost, least risk of vessel collisions during operations, and best-trained personnel always on station.
Offshore loading of ocean-going tankers and of process vessels at underwater production sites has generally required the ship to be moored and multiply connected by transfer lines to a turret around which the ship weathervanes while receiving hydrocarbon fluids from the riser. A multiport swivel joint is fixed securely to the turret and has an inlet portion which does not change in orientation to a substantial extent and an outlet portion which revolves as the vessel weathervanes to that it is always oriented toward the vessel. Examples of such structures are described in the following U.S. patents which are incorporated herein by reference: Nos. 2,894,268; 3,077,615; 3,082,440; 3,187,355; 3,236,266; 3,237,220; 3,258,793; 3,261,029; 3,430,670; 3,614,869; 4,052,090; 4,067,080; 4,107,803; 4,138,751, 4,155,670; 4,173,804 and 4,183,559.
There has been some development, however, of dynamically positioned process vessels having a multiport swivel joint as an integral part of the vessel. Typically, the swivel joint is attached to a support structure, projecting like a bowspirit from the bow or the stern, or is connected to a moonpool within the vessel. Fluid transfer lines connect the outlet portion of the swivel joint to storage facilities on the vessel. These transfer lines must be kept under substantially uniform tension and above all must not become entangled because of changes in length as the vessel weathervanes. Examples of such vessels are given in U.S. Pat. Nos. 3,335;690; 3,407,768; 3,590,407; and 3,602,302. In addition, the fluid swivels described in U.S. Pat. Nos. 4,126,336 and 4,183,559 appear to be adaptable to installation aboard a process vessel. All of these patents are incorporated herein by reference.
However, the known swivel joints, whether or not a part of the process vessel, are subject to pressures as high as 10,000 psi and contain a variety of petroleum and hydraulic fluids having a wide range of pressures, viscosities, and corrosiveness. They are, consequently, likely to develop leaks to the environment or from one fluid to another. Natural gas, because of its typically high pressures and high fluidities, often makes severe demands on the seal systems of swivel joints and is better adapted to other types of rotating interfaces between inlet hoses and transfer hoses. Moreover, a multiport swivel joint which is handling fluids is generally not adapted for handling electrical control and power lines which are as susceptible to entanglement as the fluid transfer lines because of the weathervaning of the vessel about the inlet portion of the swivel joint.
It is accordingly an object of this invention to provide a process vessel having a capacity for installing rigid pipe and control elements employed in the lower riser section.
It is also an object to provide a new vessel which is adapted to assemble flexible flowlines and form a flexible flowline bundle which is suspended at both ends beneath the vessel.
It is further an object to provide a vessel which is useful for connecting the intake end of the bundle to the buoyed riser section.
It is still further an object to provide a vessel which is adapted for servicing the flowline bundle and the riser section.
It is additionally an object to provide a vessel which is capable of disconnection from the flowline bundle under emergency conditions.